gpg: Reject signatures made with MD5.
* g10/gpg.c: Add option --allow-weak-digest-algos. (main): Set option also in PGP2 mode. * g10/options.h (struct opt): Add flags.allow_weak_digest_algos. * g10/sig-check.c (do_check): Reject MD5 signatures. * tests/openpgp/defs.inc: Add allow_weak_digest_algos to gpg.conf.